Gravity Limits Organism Size

According to the DK documentary Eyewitness -- Life (VHS), Galileo correctly calculated that the tallest Earthling lifeforms can get is ~300'. The tallest Sequoia's are almost exactly that tall.

I'm told this has to do with the limits of Capillary Action, which allows organisms to pump water up to that height.

PARALLEL: According to the National Geographic documentary Naked Science -- Earth's Crust (TV), the tallest possible mountain on Earth is only 45,000'. Any taller, and Earth's gravity creates such pressures beneath its base, that the crust melts, and the mountain sinks back down.

But, on Mars, whose gravity only around 1/3 that of Earth, Olympus Mons is 90,000' tall, 3x that of Mt. Everest.

CONCLUSION: Earth-like lifeforms, limited by gravity, could grow taller on smaller worlds, but would be shorter on larger worlds. On Mars, w/ ~1/3g, Seqoia's could grow to nearly 900'. But, on a 2.0g planet, the tallest "tree-forms" could only grow to ~150'.

I disagree with some of the specifics of your post (water transport in trees is more complex than pure capillary action, for example), but it is definitely true that 'g' (or its otherworldly equivalent) is involved in setting certain length scales.

I disagree with some of the specifics of your post (water transport in trees is more complex than pure capillary action, for example), but it is definitely true that 'g' (or its otherworldly equivalent) is involved in setting certain length scales.

Would you please be more (mathematically) specific, or point me in the right direction ?

According to the DK documentary Eyewitness -- Life (VHS), Galileo correctly calculated that the tallest Earthling lifeforms can get is ~300'. The tallest Sequoia's are almost exactly that tall.

I'm told this has to do with the limits of Capillary Action, which allows organisms to pump water up to that height.

PARALLEL: According to the National Geographic documentary Naked Science -- Earth's Crust (TV), the tallest possible mountain on Earth is only 45,000'. Any taller, and Earth's gravity creates such pressures beneath its base, that the crust melts, and the mountain sinks back down.

But, on Mars, whose gravity only around 1/3 that of Earth, Olympus Mons is 90,000' tall, 3x that of Mt. Everest.

CONCLUSION: Earth-like lifeforms, limited by gravity, could grow taller on smaller worlds, but would be shorter on larger worlds. On Mars, w/ ~1/3g, Seqoia's could grow to nearly 900'. But, on a 2.0g planet, the tallest "tree-forms" could only grow to ~150'.

Widdekind, don't overlook Dave's last post; it's critical. In the simplest terms, the structural integrity of an object increases as the square of its size, while the mass increases as the cube. "The Attack of the 50' Woman" could never happen because her leg bones would shatter if she tried to stand up.

But doesn't the square-cube law restrict heights to well under ~300' ? You yourself said, "50' woman".

Yes it does, which is why it may be critical to the OP (though he may not realize it yet). The square-cube limit is more restrictive than the relatively more forgiving capillary action limit. If the OP were looking at the maximum size of a living thing, he would misoverestimate its likely size judging on capillary action alone.

As to why they predict different things, the answer lies in the application. Square-cube applies more to motile things, things that require flexible legs for movement. The larger a motile thing (i.e. animal) gets, the larger the cross-section of its legs must be (which is why a bird has skinny legs but and elephant has great stumps for legs), At some point in scaling up, your legs would not fit under the body; you could only have a giant, single pseudopod-that's-no-longer-a-leg. This is a direct, emergent consequence of the square-cube law.

A tree is an idealized form of creature: a single leg (i.e. trunk) that is as large (actually, larger) in cross-section than its body. All its mass is situated directly above its trunk, not in a spheroid shape as with animals. And, in reaching that height, it has lost its motility (or would have, if were an animal ... and had motility).

The square-cube relationship is far more important than just structurally. Organisms get air, sunlight and often nutrients in proportion to their surface area, but use them in proportion to their volume. This puts a limit on organism size independent of gravity - and if one looks at the very largest organisms (e.g. Pando), they tend to be very sparse as a strategy to get their surface areas up.

In fact, gravity only imposes a limit on an organism's height, not size. Furthermore, the "limit" is both a function of gravity and the organism's construction. The bug-people of the planet Mongo, where there are no vertebrates, might well calculate the the largest animal on earth can't be much larger than a giant scarab.

The square-cube relationship is far more important than just structurally. Organisms get air, sunlight and often nutrients in proportion to their surface area, but use them in proportion to their volume. This puts a limit on organism size independent of gravity

In fact, gravity only imposes a limit on an organism's height, not size.

I don't know why you say this. Size (volume) is proportional to mass. Whether an organism is tall or not, its volume is still limited because its mass is limited, and its mass is limited because its limbs can't take the weight.

Furthermore, the "limit" is both a function of gravity and the organism's construction. The bug-people of the planet Mongo, where there are no vertebrates, might well calculate the the largest animal on earth can't be much larger than a giant scarab.

The point though is that gravity is a limit imposed virtually independent of other factors (such as construction); while construction will also tend to limit things, the list of possible limits endemic to a particular organism-type is virtually without end (for example, availability of food, expulsion of waste heat, etc.).

I suppose I see your point though. The wavy-tendril people of Zaxxon IV have limbs that easily exceed a thousand feet, since their limbs are gossamer-light.

I don't know why you say this. Size (volume) is proportional to mass. Whether an organism is tall or not, its volume is still limited because its mass is limited, and its mass is limited because its limbs can't take the weight.

I am thinking of organisms like Pango, which are broad but not very tall. Each stem/trunk has a height limit, but there is no gravitational limit to how many stems the root system can have.

I am thinking of organisms like Pango, which are broad but not very tall. Each stem/trunk has a height limit, but there is no gravitational limit to how many stems the root system can have.

Exactly what I was thinking; you could have a multi-ton millipede, so long as each leg only supports a thousandth of the weight. And of course, floating or semi-floating plants like kelp would not be limitted by gravity at all, so long as they retain near-nuetral bouyancy.

Cappilary action doesn't have to be a limit, either. As has been mentioned, the world's tallest tree is considerably taller than predictions would have indicated. What has not been mentioned is that, when researchers examined the top leaves of tha tree, they were suprised to find the leaves fat. That means they were filled with water, not "barely surviving" on just a trickle. As far as anyone can tell, there is no reason the tree will stop growing taller.

But this should not come as a suprise; early mining efforts proved that the so-called "limits" to pumping water to great heights can be worked around. I suspect this tree uses a strategy similar to that employed by mining engineers; pump the water as high as you can, then deposit it in a resevoir (microscopic, in the tree's case). Use the resevoir as your new "zero" level, and repeat the process.

-That's only my guess, of course; the experts are still doing some head-scratching as to how this is actually accomplished.